In measurement and instrumentation systems, it is frequently necessary to observe or measure high frequency signal waveforms. In oscilloscopes, the waveforms are displayed on a cathode ray tube for evaluation by an observer. In measurement systems, the waveforms are evaluated as to amplitude, risetime, overshoot, pulse width and the like. For extremely fast waveforms having risetimes on the order of nanoseconds or less, conventional measurement techniques are inadequate, and sampling techniques have been utilized. In one type of sampling, one sample of the test waveform is taken during each cycle. On successive cycles, samples are taken at progressively later equivalent time points in the cycle until samples of a complete cycle are taken. Then the process is repeated. The samples can be used to illuminate points on an equivalent time display on an oscilloscope. In measurement systems, the samples of equivalent time points can be digitized and stored for later processing. For example, the digitized samples may be compared with upper and lower limits to evaluate performance.
One common sampling system utilizes an error-sampled feedback loop. The test signal is sampled at a prescribed equivalent time point during its cycle. The sample is stored in an analog memory. The output of the analog memory is a linear function of the test signal amplitude at the equivalent time point. In a feedback arrangement, the memory output is fed back to the sampling input and, on successive samples, the difference between the test signal and the memory output are supplied to the input of the memory for summation. As a result, the loop output is required to change only by the difference between the instantaneous signal sample and the previous signal sample, rather than from ground or an arbitrary reference level.
The product of all gain and attenuation stages in both the forward and feedback paths of the loop is the "loop gain." The loop gain is usually about unity, while the closed loop forward gain may have any desired value. When the loop gain in the error-sampled feedback loop is exactly unity, the loop output is an accurate representation of the signal sample. In practice however, the loop gain may not be exactly unity due to drift, component tolerances and the like. In addition, the loop is frequently operated at less than unity loop gain to reduce the effect of random noise on the measured signal. Furthermore, the transient response characteristics of the loop can result in loop output errors. The above factors give rise to errors in the measured signal sample relative to the actual signal amplitude. For certain applications such as oscilloscope displays, small errors may be acceptable. In other applications, such as measurement systems which must accurately evaluate the test signal, it is desirable to minimize errors introduced by the measurement system, so that any variations can be attributed to the signal under test.
It is a general object of the present invention to provide improved sampling systems utilizing an error-sampled feedback loop.
It is another object of the present invention to provide methods and apparatus for reducing errors in sampling systems utilizing an error-sampled feedback loop.
It is a further object of the present invention to provide methods and apparatus for reducing errors in a sampling system by taking multiple samples at the same equivalent time point on successive cycles of the waveform under test, allowing the sampling loop to servo to final value.